Practically all of the manual densitometers available at the present time are equipped with a more or less elaborately designed measuring head lowering device, which makes it possible to bring the measuring head from a rest position in which the instrument may be positioned in the location desired, into a working position wherein the measuring process takes place. Obviously, the measuring head must be moved in the process at a greater or lesser distance from the support carrying the object of the measurement, and parallel to it. With the known densitometers this always requires the actuation of a handle or at least the application of a force in a direction parallel to the support, frequently resulting in an unintentional displacement of the densitometer, which in view of its slight extent often remains undetected. This may lead to incorrect positioning and thus to erroneous measurements.
U.S. Pat. No. 3,734,630 describes a special densitometer suitable for manual operation and equipped with a measuring head displaceable parallel to the support surface. The measuring head is driven by means of a linkage, not described in detail, by the application of pressure perpendiculary to the support surface to a yoke in the housing, again not explained in detail. In the case of this densitometer the risk of positioning errors is somewhat less, but it remains capable of improvement in many respects. In particular, the primitive drive of the measuring head indicated is not free of jolts and thus cannot provide the precision required at the present time, and furthermore, the densitometer is not suitable for measurements in several color channels.
Further photoelectric measuring instruments with moving measuring heads and other parts of the measuring setup are described in U.S. Pat. Nos. 3,535,046; 3,777,163; 4,173,416; 2,807,980 and 3,698,819. However, none of these references discloses a manual densitometer resembling the instrument according to the invention in any way.